In sample analysis instrumentation, and especially in separation systems such as gas or liquid chromatography and capillary electrophoresis systems, smaller dimensions will generally result in improved performance characteristics and at the same time result in reduced production and analysis costs. In this regard, miniaturized planar devices provide more effective system design and result in lower overhead due to decreased instrumentation sizing. Additionally, miniaturized planar devices enable increased speed of analysis, decreased sample and solvent consumption and the possibility of increased detection efficiency.
Several approaches towards miniaturization have developed in the art. The conventional approach provides etched planar devices on glass, silica, metal, or ceramic substrates of moderately small size. For example, planar devices may be etched in a wafer that receives a superimposed cover plate. In some approaches, certain fluid handling functions have not been integrated in the planar device and accordingly must be effected by use of conventional devices, such as fused silica capillary tubing, that are attached to the planar device. More recent approaches have used micromachining of silicon substrates and laser ablation of organic nonmetallic substrates to provide structures of much smaller size (i.e., microstructures) on the substrate. For example, there has been a trend towards providing planar systems having capillary separation microstructures. See, for example: Karasek, U.S. Pat. No. 3,538,744; Terry et al., U.S. Pat. No. 4,474,889; Goedert, U.S. Pat. No. 4,935,040; Sethi et al., U.S. Pat. No. 4,891,120; Shindo et al, U.S. Pat. No. 4,905,497; Miura et al., U.S. Pat. No. 5,132,012. See, also, attempts at miniaturization with respect to: gas chromatography (Widmer et al. (1984) Int. J. Environ. Anal. Chem. 18:1), high pressure liquid chromatography (Muller et al. (1991) J. High Resolut. Chromatogr. 14:174; Manz et al. (1990) Sensors & Actuators B1:249; Novotny et al., eds. (1985) Microcolumn Separations: Columns, Instrumentation and Ancillary Techniques (J. Chromatogr. Library, Vol. 30); Kucera, ed. (1984) Micro-Column High Performance Liquid Chromatography, Elsevier, Amsterdam; Scott, ed. (1984) Small Bore Liquid Chromatography Columns: Their Properties and Uses, Wiley, N.Y.; Jorgenson et al. (1983) J. Chromatogr. 255:335; Knox et al. (1979) J. Chromatogr. 186:405; Tsuda et al. (1978) Anal. Chem. 50:632) and capillary electrophoresis (Manz et al. (1992) J. Chromatogr. 593:253; Manz et al. Trends Anal. Chem. 10:144; Olefirowicz et al. (1990) Anal. Chem. 62:1872; Second Int'l Symp. High-Perf. Capillary Electrophoresis (1990) J. Chromatogr. 516; Ghowsi et al. (1990) Anal. Chem. 62:2714.
Micromachining techniques applied to silicon utilize a number of established techniques developed by the microelectronics industry involving micromachining of planar materials, such as silicon. Micromachining silicon substrates to form miniaturized separation systems generally involves a combination of film deposition, photolithography, etching and bonding techniques to fabricate three-dimensional microstructures. Silicon provides a useful substrate in this regard since it exhibits high strength and hardness characteristics and can be micromachined to provide structures having dimensions in the order of a few micrometers. Examples of the use of micromachining techniques to produce miniaturized separation devices on silicon or borosilicate glass chips can be found in U.S. Pat. No. 5,194,133 to Clark et al.; U.S. Pat. No. 5,132,012 to Miura et al.; in U.S. Pat. No. 4,908,112 to Pace; and in U.S. Pat. No. 4,891,120 to Sethi et al.; Fan et al., Anal. Chem. 66(1):177-184 (1994); Manz et al., Adv. Chrom. 33:1-66 (1993); Harrison et al., Sens. Actuators, B10 (2):107-116 (1993); Manz et al., Trends Anal. Chem. 10 (5):144-149 (1991); and Manz et al., Sensors and Actuators B (Chemical) B1 (1-6):249-255 (1990).
A drawback in the silicon micromachining approach to miniaturization involves the chemical activity and chemical instability of silicon dioxide (SiO.sub.2) substrates, such as silica, quartz or glass, which are commonly used in systems for both capillary electrophoresis (CE) and chromatographic analysis systems. More particularly, silicon dioxide substrates are characterized as high energy surfaces and strongly adsorb many compounds, most notably bases. The use of silicon dioxide materials in separation systems is further restricted due to the chemical instability of those substrates, as the dissolution of SiO.sub.2 materials increases in basic conditions (at pH greater than 7.0).
Accordingly, Kaltenbach et al., in commonly-assigned U.S. Pat. No. 5,500,071, and Swedberg et al., in commonly-assigned U.S. Pat. No. 5,571,410 disclose a miniaturized total analysis system comprising a miniaturized planar column device for use in a liquid phase analysis system. The miniaturized column device is provided in a substantially planar substrate, wherein the substrate is comprised of a material selected to avoid the inherent chemical activity and pH instability encountered with silicon and prior silicon dioxide-based device substrates. More specifically, a miniaturized planar column device is provided by ablating component microstructures in a substrate using laser radiation. The miniaturized column device is described as being formed by providing two substantially planar halves having microstructures thereon, which, when the two halves are folded upon each other, define a sample processing compartment featuring enhanced symmetry and axial alignment.
However, although the foregoing techniques are useful in the fabrication of miniaturized planar devices for effecting fluid handling functions in sample analysis systems, there are significant disadvantages to the prior art approaches. One significant problem remains in providing exact alignment of complementary pairs of microstructures that are respectively provided in a planar substrate and its cover plate, or in a pair of planar substrates, when such microstructures are intended to be superimposed so as to subsequently be capable of performing a fluid handling function in a unitary assembly.
For some applications, prior art planar technology has not produced a sufficient degree of alignment between the superimposed microstructures. For example, and with reference to FIG. 1A, first and second substrates 51, 52 are each shown to include spaced channels 53, 54 that are etched or otherwise formed in respective surfaces 55, 56. Superposition and appropriate bonding of the surfaces 55, 56 is intended to result in exact alignment of the channels 53, 54 such that respective fluid handling channels 61-65 are created, each of which are intended to exhibit a uniform, consistent cross-section along the major axis of the channel. However, and as illustrated, the shortcomings of the prior art result in channels on one substrate that are subject to variation in their location with respect to their complementary channels on a second, complementary substrate, thus evidencing misalignment of the channels when the substrates are superimposed. The misalignment is sufficient such that the resulting conduits (such as conduits 62-64) are subject to substantial irregularity, and in extreme cases (such as exemplified by conduit portions 65, 66) the channels are not fully integrated.
As shown in FIG. 1B, first and second substrates 71, 72 are each shown to include spaced channels 73, 74 that are etched or otherwise formed in respective surfaces 75, 76 by conventional techniques. Superimposition and appropriate bonding of the surfaces 75, 76 may succeed in adequate alignment of the channels 73, 74 such that respective fluid handling conduits 81-83 are created. However, deficiencies in many of the conventional techniques for forming the channels 73, 74 can result in edge effects and other asperities that create undesirable defects 78 in the channels 81-83. These defects 78 retard fluid flow and create localized reservoirs of fluid; accordingly, the defects 78 degrade the efficiency and uniformity of fluid flowing in the channels 81-83; the defects 78 also degrade the separation efficiency of a channel that is used to construct a separation column.
Another significant problem arises in the attempt to effect hermetic sealing of the superimposed surfaces 75, 76. This step is generally carried out using adhesives which may not fully isolate the conduits 81-83, thus resulting in cross-conduit leakage. Conventional surface bonds may be prone to failure, leakage, or to degradation induced by adverse conditions, such as high temperature environments, or by the destructive nature of certain gases or liquids that may be present in the channels.
Further, silicon substrates, and most ablatable materials such as polyimides, do not offer a sufficient combination of thermal and mechanical characteristics that otherwise would make the substrate as useful in certain applications as the named alternative materials. For instance, silicon materials are not ductile and cannot be folded, shaped, etc.; ablatable materials exhibit a low coefficient of thermal conductivity and are not susceptible to rapid and uniform heating or cooling, nor do they offer sufficient strength or ductility such that an ablatable substrate may be configured as a connecting member, housing, or support for other components in a sample analysis system. Furthermore, ablatable materials are expressly selected for their propensity to ablate upon the application of heat, and thus are not considered to be as robust and impervious to adverse (e.g., high-temperature) environments in comparison to metals and metal alloys.